Process for the production of heat- and corrosion-resistant porous metal body

ABSTRACT

A process for the production of a heat- and corrosion-resistant porous metal body which is made of a metal or whose surface layer is made of a metal, in which the porous metal body can be alloyed with chromium or a combination of chromium and aluminum uniformly throughout the entire thereof by adjusting the alloy composition in the following manner, i.e., (A) heat-treating the porous metal body in a mixed gas comprising a gas, generated by heating a powdered mixture comprising chromium or its compound and NH 4  Cl at 950° to 1100° C., and a diluent reducing gas at 800° to 1100° C, or (B) heat-treating the porous metal body in a mixed gas comprising a gas, generated by heating a powdered mixture comprising aluminum or its compound, chromium or its compound and NH 4  X, wherein X is I, F, Cl or Br, at a weight ratio of the chromium or its compound to the aluminum or its compound of 10 to 80 in terms of Cr and Al respectively at 950° to 1100° C., and a diluent reducing gas at 800° to 1100° C.

PRIOR APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/491,416 filed Jun. 16, 1995, now U.S. Pat. No. 5,672,387.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the production of a heat-and corrosion-resistant porous metal body which has continuous pores andwhich is utilizable as various filters and catalyst supports,particularly those improved in the resistances to corrosion and heat.

2. Description of the Prior Art

There have been known two types of processes for producing a porousmetal body having fine continuous pores, i.e., (1) one described inJapanese Patent Laid-Open Nos. 255686/1989 and 81767/1988 whichcomprises making a metal deposit by electroplating on a conductivenonwoven fabric or a nonwoven fabric or three-dimensional networkstructure to which electrical conductivity has been imparted by sometreatment, removing the core material of the nonwoven fabric or the likeby heat treatment, and densifying the resulting metal texture and whichis employed in, e.g., the production of a porous pure nickel body usefulas a material of an electrode for a battery; and (2) one described inJapanese Patent Publication Nos. 13077/1967 and 42703/1979 whichcomprises forming metal fibers made by drawing, cutting or the like,into a nonwoven-fabric-like structure and sintering the structure andwhich is employed in, e.g., the production of a porous stainless steelbody useful as various filters. The first type of processes (1) aredisadvantageous in that the metal species which can be deposited byelectroplating are limited. Therefore, the processes fail in making aporous metal body of Ni--Cr--Al alloy having superior corrosionresistance and heat resistance, as proposed by the applicant of thepresent invention (in Japanese Patent Laid-Open No. 206255/1993) or aporous metal body of Fe--Cr--Al alloy which has been studied as asupport material for the catalyst for treating the exhaust gas from agasoline automobile, so that any porous metal body having heatresistance enough to withstand a temperature of as high as 500° to 600°C. and high corrosion resistance cannot be obtained by the processes.The second type of processes (2) also fail in obtaining a porous metalbody having heat resistance enough to withstand a temperature of as highas 500° to 800° C. and high corrosion resistance, because it isimpossible owing to the restrictions in the technique for making metalfibers of an Ni--Cr--Al or Fe--Cr--Al alloy.

In order to overcome the disadvantages of the above two types ofprocesses, attempts have been made to employ a process for adjusting thecomposition of an alloy with a powdered material, which has been knownas a technique for anticorrosion coating of automobiles or the like andis called "diffusion coating", simultaneously with the above process.This process is one which comprises heat-treating a porous metal body at900° to 1100° C. in a state buried in powders comprising aluminum,chromium and ammonium chloride and by which both aluminum and chromiumare deposited on the surface of the body and diffused into the body toattain an alloy composition satisfying the requirements of high heat andcorrosion resistances. Such a technique is generally called"chromium-aluminizing". Further, when chromium and ammonium chloride areused as the above-mentioned powders, the technique is called"chromizing". However, the conventional techniques have a problem thatpart of the starting powdered material remains in a state adherent tothe surface of a filter or in a state sintered on the surface thereof,which is causative of clogging the filter to lower the performance ofthe filter. Therefore, there has been studied a process for preventingthe retention of a powdered material on a filter by using the powderedmaterial as a gas source and separating the filter from the powderedmaterial. A filter generally has a complicated shape. On the other hand,a nickel or iron alloy is deteriorated in toughness to result in poorworkability when an alloying amount with aluminum is increased, thoughaluminum is a component effective in improving the heat resistance.Accordingly, a porous nickel or iron metal body itself must be preformedinto a predetermined shape and thereafter adjusted so as to finallyattain a desired alloy composition. Therefore, depending on the shape ofthe porous metal body, it is necessary to use a technique by which bothchromium and aluminum can be uniformly diffused and permeated into theporous metal body placed 10 to 20 cm apart from the powdered material.However, an excess of aluminum tends to deposit on the metal body owingto the vapor pressure of chromium lower than that of aluminum.Especially, when a porous metal body having a size of as large as 10 to20 cm was subjected to the chromium-aluminizing, the aluminum alloyingcontent increased as the distance from the gas source of the powderedmaterial increased, so that in some portion the aluminum alloyingcontent was higher than 10 times that of the portion near the powder tobe diffused. On the contrary, the chromium content decreased as thedistance from the gas source of the powdered material increased, so thatin some portion the chromium content was lower than one-tenth of that ofthe portion near the powder to be diffused. The portion thus formedhaving a higher aluminum alloying content exhibited poor toughness, sothat the resulting body exhibited poor workability into a final shapeand poor antivibration properties.

As described above, the diffusion coating of a porous metal body forimparting heat and corrosion resistances thereto according to thechromium-aluminizing of the prior art had a problem that the resultingalloyed body was poor in toughness owing to its uneven distributions ofaluminum and chromium contents in the body. In particular, when a porousmetal body having a size of as large as 10 to 20 cm was alloyed withboth aluminum and chromium by gas diffusion coating using a conventionalpowdered material comprising aluminum and chromium, there occurred aproblem that an excess of aluminum deposited on some portion of the bodylocally, so that the aluminum content in the portion distant from thepowdered material as the gas source became uneven, thereby resulting ina poor toughness.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems of theprior art and aims at providing a process for producing a heat- andcorrosion-resistant porous metal body which enables uniform alloying ofa porous metal body with chromium alone or both aluminum and chromium.

The inventors of the present invention have made intensive studies andhave found that the foregoing problem could be eliminated by improvingthe heat treatment of the diffusion coating. The present invention hasbeen accomplished on the basis of this finding.

Namely, the present invention provides a process for the production of aheat- and corrosion-resistant porous metal body, which comprisesadjusting an alloy composition of a porous metal body which is made of ametal or whose surface layer is made of a metal, in which the alloycomposition is adjusted by:

(A) heat-treating the porous metal body in a mixed gas comprising a gas,generated by heating a powdered material comprising chromium or itscompound and NH₄ Cl at 950° to 1100° C., and a reducing diluent gas at800° to 1100° C., or

(B) heat-treating the porous metal body in a mixed gas comprising a gas,generated by heating a powdered material comprising aluminum or itscompound, chromium or its compound and NH₄ X, wherein X is I, F, Cl orBr, at a mixing weight ratio (Cr/Al) of the chromium or its compound tothe aluminum or its compound of 10 to 80 in terms of Cr and Alrespectively at 950° to 1100° C., and a reducing diluent gas, at 800° to1100° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the heat treatment apparatus used fordiffusion coating in Examples.

FIGS. 2a, 2b and 2c are explanation diagrams showing the heat cycle whena heat-up and cool-down cycle is repeated in the heat treatmentaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The porous metal body to be subjected to the above heat treatmentaccording to the present invention may be constituted of iron, nickel,cobalt, an alloy based on one or more of them, or the like.

The powdered material to be used as the gas source in the presentinvention may be any known one used in the chromium-aluminizingaccording to diffusion coating. In other words, as the powderedmaterial, there may be used aluminum and chromium or an aluminumcompound, e.g. aluminum halide (such as AlF₃, AlCl₃, AlBr₃ or AlI₃) anda chromium compound, e.g., chromium halide (such as CrF₂, CrCl₂, CrBr₂or CrI₂), and a halide activator (NH₄ X, wherein X is I, F, Cl or Br).Further, alumina or the like may be added for the purpose of preventingthe sintering of powder particles with each other when the powderedmaterial is heated. When alumina is used in the powdered material forthis purpose, the aluminum content of this alumina is excluded from theforegoing mixing weight ratio (Cr/Al) because the alumina is not asource component for alloying.

According to the present invention, a reducing gas such as hydrogen or amixture thereof with an inert gas such as argon or neon is preferablyused as a diluent gas with which a gas generated by heating the abovepowdered material at 950° to 1100° C. is diluted. In particular, areducing gas such as hydrogen can form a reducing atmosphere, which iseffective in preventing the initial thermal oxidation of a metal whichhinders the alloying of a porous metal body with chromium and aluminum.Therefore, it is preferable that the gas mixture contain a reducing gassuch as hydrogen in an amount of at least 60% by volume, stillpreferably at least 80% by volume. The reducing diluent gas ispreferably introduced into the heat treatment zone and mixed thereinwith a gas generated by heating the above powdered material to adjustthe concentrations of aluminum and chromium or those of aluminumcompound and chromium compound to desired values through dilution. Theadjustment of the concentration of aluminum and chromium with a diluentgas is effective in uniforming the concentrations of the reactants,i.e., aluminum and chromium, throughout the treatment atmosphere gas ascompletely as possible.

In the diffusion coating of the prior art wherein a powdered materialcomprising the components to be diffused into a porous metal body isused as the gas source, the chromium concentration rapidly lowers as thedistance from the powdered material increases, owing to the vaporpressure of chromium lower than that of aluminum. Therefore, the portionof a porous metal body distant from the powdered material is alloyedwith an excess of aluminum to result in poor toughness, not satisfyingthe requirements of excellent workability into a desired shape andantivibration properties. According to the present invention, thepressure of the aluminum vapor generating from the starting powderedmaterial is lowered to thereby increase the pressure of chromium vaporrelatively to that of aluminum vapor, by which the deposition of anexcess of aluminum on a porous metal body is inhibited. Further, asdescribed above, the concentrations of aluminum and chromium are highlyuniformed by mixing the gas generated from the powdered material with adiluent gas. It is desirable that the powdered material contain aluminum(or an aluminum compound) and chromium (or a chromium compound) at aCr/Al mixing ratio (by weight) of the chromium (or its compound) to thealuminum (or its compound) of 10 to 80, more desirably 15 to 70 (interms of Cr and Al). The heat treatment for diffusion coating ispreferably conducted at 800° to 1100° C. for 3 to 8 hours.

According to the present invention, as described above, the Cr/Al mixingratio of the powdered material is enhanced and the component gases to bediffused are mixed with a reducing diluent gas, by which the aluminumcomponent is suppressed and the concentration of chromium in theatmosphere gas is adjusted so as to be uniform throughout the entireheat treating zone. In order to adjust the concentration of thiscomponent gas of chromium or a chromium compound to be diffused to amore suitable value to thereby uniform the chromium content of the alloyformed on the surface of a porous metal body more completely, it ispreferable that the reducing gas be used in an amount of 0.01 to 0.06mol/min per 1 kg of the chromium or its compound powder in terms of Crand the chromium or chromium compound in the powdered material as thegas source be in an amount of 15 to 35% by weight, still preferably 20to 35% by weight (in terms of Cr) based on the whole powdered material,with the proviso that the Cr/Al ratio of the powdered material lieswithin the above range. When the amount of the reducing gas is less than0.01 mol/min, the concentration of a more reactive species, i.e., acomponent gas of aluminum (or an aluminum compound) in the treatmentatmosphere gas will increase to alloy a porous metal body with too muchaluminum, which will lead to the production of a porous metal bodyhaving a nonuniform composition and poor toughness. On the contrary,when the amount of the reducing gas exceeds 0.06 mol/min, the chromiumconcentration will be too low to alloy a porous metal body with enoughchromium, so that the resulting body will not be sufficiently improvedin heat and corrosion resistances, though the concentration of aluminumor an aluminum compound gas is suppressed. When chromium or a chromiumcompound is in an amount of less than 15% by weight (in terms of Cr)based on the whole powdered material, the concentration of chromium or achromium compound in the treatment atmosphere gas will be too low togive a porous metal body alloyed with a sufficient amount of chromium,while when the content as Cr exceeds 35% by weight, the concentration ofchromium or its chromium compound gas in the atmosphere gas near thepowder as the gas source will be too high to obtain a porous metal bodyhaving a uniform alloy composition. Thus, both cases are unfavorable.

In the preferred embodiment of this invention, the above heat-treatingstep for the adjustment of the composition of the formed alloy may beperformed by repeating a cycle comprising temperature rise andtemperature fall. In such a manner, the chromium content can besufficiently compensated. The inventors of the present invention havedirected their attention to the fact that chromium deposits from a gassupersaturated with chromium vapor by temperature fall, and haveattained the accelerated deposition of chromium by repeating thetemperature fall.

The temperature fall need not be one reaching room temperature as shownin FIG. 2 (a), but may be performed in a heat cycle pattern comprisingfall to a certain temperature and re-rise to the treatment temperatureas shown in FIG. 2 (b). The temperature fall is preferably carried outto reach a range of 800°-900° C. As properties required for use in afilter, the chromium content may lie within such a range that the heatand corrosion resistances are not be deteriorated, preferably rangesfrom 15 to 35% by weight.

It is preferable from the standpoint of industrial productivity that thefrequency of such a heat cycle be as small as possible, because a higherfrequency of the heat cycle leads to a lower efficiency and a highertreatment cost. The frequency thereof to attain the lowest necessarychromium content is preferably 2 or 3. As described above, the processof the present invention makes it possible to provide a uniform chromiumcontent even in the internal region of a porous metal body in itsthicknesswise direction by repeating the heat cycle to therebypositively utilize the phenomenon that chromium deposits by thetemperature fall. Further, according to the present invention, thecontents of aluminum and chromium in the resulting alloyed body can beadjusted by single heat treatment. Since this technique makes itpossible to provide a uniform composition of aluminum and chromium to aporous metal body, which has been formed into a desired shape, in thethickness direction of the body, the thus treated porous metal body hasheat and corrosion resistances even in the inner area. Consequently, themetal body can exhibit heat resistance and corrosion resistance at 700°C. or higher.

The process of the present invention can be applied to chromizing usingonly a chromium component without any aluminum component and this isalso included within the present invention. In this case, theheat-treatment for diffusion coating is carried out preferably at 950°to 1100° C. for 3 to 8 hours.

In the chromizing process, the amount of a reducing gas such as hydrogenis preferably 0.01 to 0.06 mol/min per 1 kg of the chromium or chromiumcompound in terms of Cr, while the chromium or its compound as the gassource is preferably in an amount of 15 to 35% by weight, stillpreferably 20 to 35% by weight in terms of Cr based on the wholepowdered material. When the amount of the reducing gas is less than 0.01mol/min, the concentration of the gas of chromium or its chromiumcompound in the treatment atmosphere gas will be so nonuniform as togive a porous metal body having a nonuniform composition unfavorably. Onthe contrary, when the amount of the reducing gas exceeds 0.06 mol/min,the concentration of the gas of chromium or its chromium compound willbe too low to give a porous metal body with a sufficient amount ofchromium, so that the resulting body will not sufficiently be improvedin heat and corrosion resistances. Further, when the content, in termsof Cr, of chromium or chromium compound powder is less than 15% byweight, the concentration of chromium or its chromium compound in thetreatment atmosphere gas will be too low to give a porous metal bodywith a sufficient amount of chromium, while when the content, in termsof Cr, of powdered chromium or its compound exceeds 35% by weight, theconcentration of the gas of chromium or its chromium compound in theatmosphere gas near the gas source will be too high. Both cases areunfavorable. In this chromizing process, the above heat-treating stepfor the adjustment of the composition of the formed alloy may beperformed by repeating a cycle comprising temperature rise andtemperature fall. In such a manner, the chromium content can besufficiently compensated. The foregoing amount (0.01 to 0.06 mol/min) ofthe reducing gas per 1 kg (in terms of Cr) of the chromium or itscompound used in the chromium-aluminizing or chromizing is the amountfor the reducing gas such as hydrogen excluding an inert gas which maybe included in the reducing diluent gas.

According to the present invention, a porous metal body can be alloyedwith both aluminum and chromium uniformly in a wide region to give aporous metal body exhibiting a heat resistance enough to withstand atemperature of as high as 500° C. or above and a high corrosionresistance, even when it is a filter having a complicated shape and asize of as large as 10 to 20 cm or above.

The processes of the present invention are applicable not only to afilter but also to a support for active components such as catalystswhich are required to be resistant to heat and corrosion.

This invention will be illustrated in more detail by the followingexamples.

EXAMPLES 1-9

A porous metal body (trade name: CELMET, nickel-base porous body ofthree-dimensional network structure, a product of Sumitomo ElectricIndustries, Ltd.) made by metallizing polyurethane by plating and havinga skeleton size of 60 μm, a pore diameter of 0.7 mm, a packing densityof 5% and a thickness of 1.8 mm was obtained. The thus obtained porousmetal bodies were wound around a cylindrical core to form five-plycylindrical porous bodies. Each body was uniformly pressed into acylindrical sample having an inner diameter of 3 cm, an outer diameterof 4.2 cm and a height of 20 cm. As shown in FIG. 1, each sample 1 wasput on a powdered material 2 including Cr, Al and NH₄ Cl, based on thetotal weight of Cr, Al, NH₄ Cl and Al₂ O₃, as shown in Table 1, andspread on the bottom of a furnace 5 (width: 17 cm, length: 23 cm,height: 25 cm) in a depth of about 2 cm. The powdered material 2 was asource for supplying component gases to be diffused into and alloyedwith the porous metal body and is hereinafter referred to as "diffusiongas source powdered material". Chromium-aluminizing was conducted in agaseous atmosphere at 1050° C. for 5 hours. In FIG. 1, referencenumerals 3 and 4 represent a terminal of a temperature control rod and areducing diluent gas, respectively. The numerical values indicatedwithin () after the flow rates of H₂ in the column "Reducing diluentgas" of Table 1 are the flow rates (mol/min) of the reducing gas (H₂)per 1 kg of Cr in the powdered material. Similarly to Table 1, the flowrates of the reducing gas in the following Examples are also shown inthe respective tables.

Thereafter, the thus treated samples were taken out of the furnace andcut in square specimens (1 cm×1 cm). Each square specimen was peeledoff, and analyzed for composition of the porous metal body alloyed withAl and Cr by ionization absorption spectrophotometry. Further, eachsample treated by chromium-aluminizing was further subjected to heatresistance test. In this test, the heat resistance of the alloyed samplewas evaluated by oxidizing the sample in the open air at 800° C. for 50hours to determine the weight gain thereof. A case wherein the weightgain was up to 10% was judged good.

Furthermore, the alloyed sample was subjected to workability test. Inthis test, a case wherein the sample did not break even when bentrectangularly three or more times was judged good. An overall judgmentwas conducted based on the results of both items. The results are givenin Table 1. In the following tables, satisfactory results are markedwith circle (o) and unacceptable results are marked with cross (x).Further, "Workability" is shown the frequency of bending until breakageoccurred in the porous metal body.

                                      TABLE 1                                     __________________________________________________________________________                         Reducing     Distance                                    Compo-    Composition of                                                                           diluent      from                                        sition    powdered material                                                                        gas          source                                      (wt. %)   (wt. %)*   (cc/min)  Heat                                                                             material                                    Ex. No.                                                                           Ni Fe Cr                                                                              Al                                                                              NH.sub.4 Cl                                                                       Cr/Al                                                                            Ar                                                                              H.sub.2                                                                          (mol/min)                                                                          cycle                                                                            (cm)                                        __________________________________________________________________________    1   100                                                                              -- 25                                                                              0.5                                                                             0.5 50 --                                                                              200                                                                              (0.027)                                                                            1  0                                           2   100                                                                              -- 25                                                                              0.5                                                                             0.5 50 --                                                                              200                                                                              (0.027)                                                                            1  10                                          3   100                                                                              -- 25                                                                              0.5                                                                             0.5 50 --                                                                              200                                                                              (0.027)                                                                            1  20                                          4   100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            1  0                                           5   100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            1  10                                          6   100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            1  20                                          7   100                                                                              -- 35                                                                              0.5                                                                             0.5 70 --                                                                              200                                                                              (0.019)                                                                            1  0                                           8   100                                                                              -- 35                                                                              0.5                                                                             0.5 70 --                                                                              200                                                                              (0.019)                                                                            1  10                                          9   100                                                                              -- 35                                                                              0.5                                                                             0.5 70 --                                                                              200                                                                              (0.019)                                                                            1  20                                          __________________________________________________________________________    Composition                                                                   after alloying  Heat resistance                                                                        Workability                                          (wt. %)         (weight gain)                                                                          (frequency                                                                           Overall                                       Ex. No.                                                                           Ni Fe Cr Al (%)      of bending)                                                                          judgment                                      __________________________________________________________________________    1   bal                                                                              -- 22 6  3        5      ∘                                 2   bal                                                                              -- 23 5  4        5      ∘                                 3   bal                                                                              -- 25 5  4        5      ∘                                 4   bal                                                                              -- 17 8  7        3      ∘                                 5   bal                                                                              -- 17 7  7        3      ∘                                 6   bal                                                                              -- 18 6  7        3      ∘                                 7   bal                                                                              -- 25 5  8        6      ∘                                 8   bal                                                                              -- 24 4  8        6      ∘                                 9   bal                                                                              -- 25 4  8        6      ∘                                 __________________________________________________________________________     *Remainder: Al.sub.2 O.sub.3 -                                           

COMPARATIVE EXAMPLES 1-3

The same treatments as that of Examples 1-3 were repeated except thatthe chromium concentration of the diffusion gas source powdered materialwas changed to 3% by weight and the flow rate (mol/min) of the reducinggas (H₂) per 1 kg of Cr was changed as shown in Table 2. The results aregiven in Table 2.

COMPARATIVE EXAMPLES 4-6

The same treatments as that of Examples 7-9 were repeated except thatthe chromium concentration of the diffusion gas source powdered materialwas changed to 50% by weight and the flow rate; (mol/min) of thereducing gas (H₂) per 1 kg of Cr was chanted as shown in Table 2. Theresults are given in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                         Reducing     Distance                                        Compo-                                                                              Composition of                                                                           diluent      from                                            sition                                                                              powdered material                                                                        gas          source                                      Comp.                                                                             (wt. %)                                                                             (wt. %)*   (cc/min)  Heat                                                                             material                                    Ex. No.                                                                           Ni Fe Cr                                                                              Al                                                                              NH.sub.4 Cl                                                                       Cr/Al                                                                            Ar                                                                              H.sub.2                                                                          (mol/min)                                                                          cycle                                                                            (cm)                                        __________________________________________________________________________    1   100                                                                              -- 3 0.5                                                                             0.5 6  --                                                                              200                                                                              (0.228)                                                                            1  0                                           2   100                                                                              -- 3 0.5                                                                             0.5 6  --                                                                              200                                                                              (0.228)                                                                            1  10                                          3   100                                                                              -- 3 0.5                                                                             0.5 6  --                                                                              200                                                                              (0.228)                                                                            1  20                                          4   100                                                                              -- 50                                                                              0.5                                                                             0.5 100                                                                              --                                                                              200                                                                              (0.014)                                                                            1  0                                           5   100                                                                              -- 50                                                                              0.5                                                                             0.5 100                                                                              --                                                                              200                                                                              (0.014)                                                                            1  10                                          6   100                                                                              -- 50                                                                              0.5                                                                             0.5 100                                                                              --                                                                              200                                                                              (0.014)                                                                            1  20                                          __________________________________________________________________________        Composition                                                                   after alloying                                                                            Heat resistance                                                                        Workability                                          Comp.                                                                             (wt. %)     (weight gain)                                                                          (frequency                                                                           Overall                                       Ex. No.                                                                           Ni Fe Cr Al (%)      of bending)                                                                          judgment                                      __________________________________________________________________________    1   bal                                                                              -- 2  12 8        1      x                                             2   bal                                                                              -- 0.5                                                                              11 9        1      x                                             3   bal                                                                              -- 0.1                                                                              11 10       1      x                                             4   bal                                                                              -- 26 3  9        6      x                                             5   bal                                                                              -- 27 1  11       7      x                                             6   bal                                                                              -- 28 0.2                                                                              15       8      x                                             __________________________________________________________________________     *Remainder: Al.sub.2 O.sub.3 -                                           

EXAMPLES 10-12

Nonwoven metal fabrics of Fe having a fiber diameter of 30 μm, packingdensity of 18% and a thickness of 0.7 mm were made by metallizingnonwoven fabrics made of organic fibers or carbon fibers with Fe byplating.

Each of the thus obtained nonwoven metal fabrics metallized with Fe byplating was wound around a cylindrical core to form a three-plycylindrical porous body. Each body was uniformly pressed into acylindrical sample having an inner diameter of 3 cm, an outer diameterof 3.3 cm and a height of 20 cm and subjected to chromium-aluminizing inthe same manner as described in Examples 1-3. Each sample was put on adiffusion gas source powdered material including Cr, Al and NH₄ Cl,based on the total weight of Cr, Al, NH₄ Cl and Al₂ O₃, as shown inTable 3, and spread on the bottom of a furnace in a depth of about 2 cm.The chromium-aluminizing treatment was conducted in a gaseous atmosphereat 1050° C. for 3 hours. Thereafter, the thus treated nonwoven metalfabric was subjected to compositional analysis, heat resistance test andworkability test, in the same testing procedures as described inExamples 1-3. The results are given in Table 3.

EXAMPLES 13-15

The same treatments as that of Examples 10-12 were repeated except thathydrogen gas and argon gas were introduced at rates of 240 cc/min and 60cc/min, respectively, in place of the reducing diluent gas used inExamples 10-12 so that the flow rate (mol/min) of the reducing gas (H₂)per 1 kg of Cr was changed as shown in Table 3. The results are given inTable 3.

EXAMPLES 16-21

The same treatments as that of Examples 10-12 were repeated except thateither one of two types of heat cycles as shown in FIG. 2(a) or FIG.2(c) was conducted on nonwoven metal fabrics of nickel and further thealuminum content of the powdered material was changed as shown in Table3. The results are given in Table 3.

The patterns of heat cycle used in Examples are shown in FIG. 2(a) to2(c). More specifically, FIG. 2(a) shows a heat cycle comprising heatingup from room temperature to the temperature predetermined for chromizingor chromium-aluminizing treatment and cooling down to room temperatureafter the treatment. FIGS. 2(b) and 2(c) show alternative heat cyclepatterns in which a cycle of heating up and cooling down are repeated.For instance, in Example 10-12, as shown in FIG. 2(a), the nonwovenfabric was subjected to heating to 1050° C., chromium-aluminizing atthat temperature for 3 hr., and then cooling to room temperature. InExample 16-18, as shown in FIG. 2(b), the nonwoven fabric was subjectedto heating to 1050° C., chromium-aluminizing at that temperature for 3hr., cooling to 800° C., reheating to 1050° C., and then cooling to roomtemperature. In Examples 19-21, as shown in FIG. 2(c), the nonwovenfabric was subjected to heating to 1050° C., chromium-aluminizing atthat temperature for 3 hr., cooling to 800° C., reheating to 1050° C.,then repeating the same cooling and heating cycle, and finally coolingto room temperature.

                                      TABLE 3                                     __________________________________________________________________________                         Reducing     Distance                                    Compo-    Composition of                                                                           diluent      from                                        sition    powdered material                                                                        gas          source                                      (wt. %)   (wt. %)*   (cc/min)  Heat                                                                             material                                    Ex. No.                                                                           Ni Fe Cr                                                                              Al                                                                              NH.sub.4 Cl                                                                       Cr/Al                                                                            Ar                                                                              H.sub.2                                                                          (mol/min)                                                                          cycle                                                                            (cm)                                        __________________________________________________________________________    10  -- 100                                                                              25                                                                              0.5                                                                             0.5 50 --                                                                              200                                                                              (0.027)                                                                            1  0                                           11  -- 100                                                                              25                                                                              0.5                                                                             0.5 50 --                                                                              200                                                                              (0.027)                                                                            1  10                                          12  -- 100                                                                              25                                                                              0.5                                                                             0.5 50 --                                                                              200                                                                              (0.027)                                                                            1  20                                          13  -- 100                                                                              25                                                                              0.5                                                                             0.5 50 60                                                                              240                                                                              (0.033)                                                                            1  0                                           14  -- 100                                                                              25                                                                              0.5                                                                             0.5 50 60                                                                              240                                                                              (0.033)                                                                            1  10                                          15  -- 100                                                                              25                                                                              0.5                                                                             0.5 50 60                                                                              240                                                                              (0.033)                                                                            1  20                                          16  100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            2  0                                           17  100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            2  10                                          18  100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            2  20                                          19  100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            3  0                                           20  100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            3  10                                          21  100                                                                              -- 25                                                                              1.5                                                                             0.5 16.7                                                                             --                                                                              200                                                                              (0.027)                                                                            3  20                                          __________________________________________________________________________    Composition                                                                   after alloying  Heat resistance                                                                        Workability                                          (wt. %)         (weight gain)                                                                          (frequency                                                                           Overall                                       Ex. No.                                                                           Ni Fe Cr Al (%)      of bending)                                                                          judgment                                      __________________________________________________________________________    10  -- bal                                                                              23 8  7        3      ∘                                 11  -- bal                                                                              24 7  7        3      ∘                                 12  -- bal                                                                              25 7  7        3      ∘                                 13  -- bal                                                                              22 6  8        6      ∘                                 14  -- bal                                                                              21 5  8        6      ∘                                 15  -- bal                                                                              21 5  8        6      ∘                                 16  bal                                                                              -- 19 6  5        5      ∘                                 17  bal                                                                              -- 19 6  5        5      ∘                                 18  bal                                                                              -- 19 6  4        5      ∘                                 19  bal                                                                              -- 23 6.5                                                                              4        5      ∘                                 20  bal                                                                              -- 23 6.5                                                                              4        5      ∘                                 21  bal                                                                              -- 25 6.5                                                                              4        5      ∘                                 __________________________________________________________________________     *Remainder: Al.sub.2 O.sub.3 -                                           

EXAMPLES 22-27

Porous metal bodies (trade name: CELMET, nickel-base porous body ofthree-dimensional network structure, a product of Sumitomo ElectricIndustries, Ltd.) made by metallizing polyurethane by plating and havinga skeleton size of 60 μm, a pore diameter of 0.7 mm, a packing densityof 5% and a thickness of 1.8 mm and an porous metal body of iron wereobtained. The thus obtained porous metal bodies were wound around acylindrical core to form five-ply cylindrical porous bodies. Each bodywas uniformly pressed into a cylindrical sample having an inner diameterof 3 cm, an outer diameter of 4.2 cm and a height of 20 cm. As shown inFIG. 1, each sample was put on a diffusion gas source powdered materialincluding Cr and NH₄ Cl, based on the total weight of Cr, NH₄ Cl and Al₂O₃,, as shown in Table 4, and spread on the bottom of a furnace (width:17 cm, length: 23 cm and height: 25 cm) in a depth of about 2 cm.Chromizing was conducted in a gaseous atmosphere at 1050° C. for 5hours.

Thereafter, the thus treated sample was taken out of the furnace and cutin square specimens (1 cm×1 cm). Each square specimen was peeled off,and analyzed for composition of the porous metal body alloyed with Cr byionization absorption spectro-photometry. Each sample treated bychromizing was further subjected to heat resistance test. In this test,the heat resistance of the alloyed sample was evaluated by oxidizing thesample in the open air at 700° C. for 50 hours to determine the weightgain thereof. A case wherein the weight gain was up to 10% was judgedgood. The results are given in Table

                                      TABLE 4                                     __________________________________________________________________________                         Reducing     Distance                                    Compo-    Composition of                                                                           diluent      from                                        sition    powdered material                                                                        gas          source                                      (wt. %)   (wt. %)*   (cc/min)  Heat                                                                             material                                    Ex. No.                                                                           Ni Fe Cr                                                                              Al                                                                              NH.sub.4 Cl                                                                       Cr/Al                                                                            Ar                                                                              H.sub.2                                                                          (mol/min)                                                                          cycle                                                                            (cm)                                        __________________________________________________________________________    22  -- 100                                                                              30                                                                              --                                                                              0.5 -- --                                                                              200                                                                              (0.023)                                                                            1  0                                           23  -- 100                                                                              30                                                                              --                                                                              0.5 -- --                                                                              200                                                                              (0.023)                                                                            1  10                                          24  -- 100                                                                              30                                                                              --                                                                              0.5 -- --                                                                              200                                                                              (0.023)                                                                            1  20                                          25  100                                                                              -- 20                                                                              --                                                                              0.5 -- --                                                                              200                                                                              (0.034)                                                                            1  0                                           26  100                                                                              -- 20                                                                              --                                                                              0.5 -- --                                                                              200                                                                              (0.034)                                                                            1  10                                          27  100                                                                              -- 20                                                                              --                                                                              0.5 -- --                                                                              200                                                                              (0.034)                                                                            1  20                                          __________________________________________________________________________    Composition                                                                   after alloying  Heat resistance                                                                        Workability                                          (wt. %)         (weight gain)                                                                          (frequency                                                                           Overall                                       Ex. No.                                                                           Ni Fe Cr Al (%)      of bending)                                                                          judgment                                      __________________________________________________________________________    22  -- bal                                                                              28 -- 8.75     7      ∘                                 23  -- bal                                                                              22 -- 9.0      7      ∘                                 24  -- bal                                                                              20 -- 10.0     8      ∘                                 25  bal                                                                              -- 24 -- 9.5      7      ∘                                 26  bal                                                                              -- 21 -- 9.3      7      ∘                                 27  bal                                                                              -- 20 -- 10.0     8      ∘                                 __________________________________________________________________________     *Remainder: Al.sub.2 O.sub.3 -                                           

EXAMPLES 28-30

Nonwoven metal fabrics of nickel having a fiber diameter of 30 μm,packing density of 18% and a thickness of 0.7 mm were made bymetallizing nonwoven fabrics made of organic fibers or carbon fiberswith nickel by plating.

Each of the thus obtained nonwoven metal fabrics of nickel was woundaround a cylindrical core to form a three-ply cylindrical porous body.Each body was uniformly pressed into a cylindrical sample having aninner diameter of 3 cm, an outer diameter of 3.3 cm and a height of 20cm and alloyed with chromium by chromizing treatment in the same manneras described in Examples 22-24. The treatment was conducted in a gaseousatmosphere at 1050° C. for 3 hours. Thereafter, the thus treatedmetallized nonwoven fabric was subjected to compositional analysis andheat resistance test in the same testing procedures as described inExamples 22-24. The results are given in Table 5.

EXAMPLES 31-33

The same treatments as that of Examples 28-30 were repeated except thathydrogen gas and argon gas were introduced at rates of 240 cc/min and 60cc/min, respectively, in place of the reducing diluent gas used inExamples 28-30; so that the flow rate (mol/min) of the reducing gas (H₂)per 1 kg of Cr was changed as shown in Table 5. The results are given inTable 5.

COMPARATIVE EXAMPLES 7-9

The same treatments as that of Examples 28-30 were repeated except thatonly argon gas was introduced at rate of 200 cc/min in place of thereducing diluent gas used in Examples 28-30. The results are given inTable 5.

                                      TABLE 5                                     __________________________________________________________________________                           Deducing      Distance                                 Compo-      Composition of                                                                           diluent       from                                     sition      powdered material                                                                        gas           source                                   (wt. %)     (wt. %)*   (cc/min)   Heat                                                                             material                                       Ni Fe Cr                                                                              Al                                                                              NH.sub.4 Cl                                                                       Cr/Al                                                                            Ar H.sub.2                                                                          (mol/min)                                                                          cycle                                                                            (cm)                                     __________________________________________________________________________    Ex. No.                                                                       28    100                                                                              -- 30                                                                              --                                                                              0.5 -- -- 200                                                                              (0.023)                                                                            1  0                                        29    100                                                                              -- 30                                                                              --                                                                              0.5 -- -- 200                                                                              (0.023)                                                                            1  10                                       30    100                                                                              -- 30                                                                              --                                                                              0.5 -- -- 200                                                                              (0.023)                                                                            1  20                                       31    100                                                                              -- 30                                                                              --                                                                              0.5 -- 60 240                                                                              (0.027)                                                                            1  0                                        32    100                                                                              -- 30                                                                              --                                                                              0.5 -- 60 240                                                                              (0.027)                                                                            1  10                                       33    100                                                                              -- 30                                                                              --                                                                              0.5 -- 60 240                                                                              (0.027)                                                                            1  20                                       Comparative                                                                   Examples                                                                       7    100                                                                              -- 30                                                                              --                                                                              0.5 -- 200                                                                              -- --   1  0                                         8    100                                                                              -- 30                                                                              --                                                                              0.5 -- 200                                                                              -- --   1  10                                        9    100                                                                              -- 30                                                                              --                                                                              0.5 -- 200                                                                              -- --   1  20                                       __________________________________________________________________________    Composition                                                                   after alloying     Heat resistance                                                                        Workability                                       (wt. %)            (weight gain)                                                                          (frequency                                                                           Overall                                          Ni Fe Cr  Al (%)      of bending)                                                                          judgment                                   __________________________________________________________________________    Ex. No.                                                                       28    bal                                                                              -- 33  -- 8.50     7      ∘                              29    bal                                                                              -- 27  -- 8.75     7      ∘                              30    bal                                                                              -- 25  -- 9.5      7      ∘                              31    bal                                                                              -- 32  -- 8.50     7      ∘                              32    bal                                                                              -- 26  -- 9.5      7      ∘                              33    bal                                                                              -- 25  -- 9.5      7      ∘                              Comparative                                                                   Examples                                                                       7    bal                                                                              -- 10  -- 18.0     8      x                                           8    bal                                                                              -- 0.5 -- 20.5     8      x                                           9    bal                                                                              -- 0.2 -- 22.5     8      x                                          __________________________________________________________________________     *Remainder: Al.sub.2 O.sub.3 -                                           

EXAMPLES 34-36

The same treatments as that of Examples 25-27 were repeated except thatthe chromium concentration of the diffusion gas source powdered materialwas changed to 15% by weight and hydrogen gas was introduced at a rateof 280 cc/min in place of the reducing diluent gas used in Examples25-27 so that the flow rate (mol/min) of the reducing gas (H₂) per 1 kgof Cr was changed as shown in Table 6. The results are given in Table 6.

EXAMPLES 37-39

The same treatments as that of Examples 28-30 were repeated except thathydrogen gas and argon gas were introduced at rates of 100 cc/min and100 cc/min, respectively, in place of the reducing diluent gas used inExamples 28-30 so that the flow rate (mol/min) of the reducing gas (H₂)per 1 kg of Cr was changed as shown in Table 6. The results are given inTable 6.

                                      TABLE 6                                     __________________________________________________________________________                         Deducing      Distance                                   Compo-    Composition of                                                                           diluent       from                                       sition    powdered material                                                                        gas           source                                     (wt. %)   (wt. %)*   (cc/min)   Heat                                                                             material                                   Ex. No.                                                                           Ni Fe Cr                                                                              Al                                                                              NH.sub.4 Cl                                                                       Cr/Al                                                                            Ar H.sub.2                                                                          (mol/min)                                                                          cycle                                                                            (cm)                                       __________________________________________________________________________    34  100                                                                              -- 15                                                                              --                                                                              0.5 -- -- 280                                                                              (0.055)                                                                            1  0                                          35  100                                                                              -- 15                                                                              --                                                                              0.5 -- -- 280                                                                              (0.055)                                                                            1  10                                         36  100                                                                              -- 15                                                                              --                                                                              0.5 -- -- 280                                                                              (0.055)                                                                            1  20                                         37  100                                                                              -- 30                                                                              --                                                                              0.5 -- 100                                                                              100                                                                              (0.011)                                                                            1  0                                          38  100                                                                              -- 30                                                                              --                                                                              0.5 -- 100                                                                              100                                                                              (0.011)                                                                            1  10                                         39  100                                                                              -- 30                                                                              --                                                                              0.5 -- 100                                                                              100                                                                              (0.011)                                                                            1  20                                         __________________________________________________________________________    Composition                                                                   after alloying  Heat resistance                                                                        Workability                                          (wt. %)         (weight gain)                                                                          (frequency                                                                           Overall                                       Ex. No.                                                                           Ni Fe Cr Al (%)      of bending)                                                                          judgment                                      __________________________________________________________________________    34  bal                                                                              -- 18 -- 10.0     8      ∘                                 35  bal                                                                              -- 17 -- 10.0     8      ∘                                 36  bal                                                                              -- 16 -- 10.0     8      ∘                                 37  bal                                                                              -- 26 -- 8.9      7      ∘                                 38  bal                                                                              -- 22 -- 9.0      7      ∘                                 39  bal                                                                              -- 20 -- 10.0     8      ∘                                 __________________________________________________________________________     *Remainder: Al.sub.2 O.sub.3 -                                           

According to the present invention, as described above, the pressure ofthe aluminum vapor generated from the powdered material is lowered toincrease the pressure of the chromium vapor relatively to that of thealuminum vapor, by which the deposition of excess aluminum onto a porousmetal body can be inhibited and the concentrations of aluminum andchromium can be highly uniformed throughout the entire treatmentatmosphere. By virtue of these effects, a porous metal body can bealloyed with aluminum and chromium uniformly throughout the entirethereof, even when it has a complicated shape or a relatively largesize. Accordingly, the present invention enables the improvement of aporous metal body in heat and corrosion resistances without generatingan area deteriorated in toughness by excess alloying with aluminum orchromium.

Further, according to the present invention, the concentrations of thecomponent gases to be diffused and permeated into a porous metal bodycan be uniformed throughout the entire heat treatment atmosphere, bywhich a porous metal body to be treated can be alloyed with chromiumuniformly throughout the entire thereof even when it has a complicatedshape or a relatively large size. Therefore, the present inventionenables the improvement of porous metal body in heat and corrosionresistances without generating an area deteriorated in toughness byexcess alloying with chromium.

What is claimed is:
 1. A process for the production of a heat- andcorrosion-resistant porous metal body having at least a surface layer ofmetal, said process comprisingplacing said body on or apart from apowder, said body not being embedded in said powder, said powdercomprising NH₄ Cl and chromium and/or a chromium compound in an amountof 15% to 35% by weight, calculated as chromium and based on saidpowder, heating said powder and said body to 800° 1100° C. in a heatingzone in the presence of a mixed gas comprising a generated gas and areducing diluent gas, said generated gas resulting from heating saidpowder to 950° C. to 1100° C., and said reducing diluent gas beingflowed into said heating zone substantially throughout said heating inan amount of 0.01 to 0.06 mol/min per 1 kg, in terms of Cr, of saidchromium or chromium compound.
 2. A process as set forth in claim 1,wherein at least two runs of a heat cycle comprising temperature riseand temperature fall are included in said heating of said powder andsaid body.
 3. A process as set forth in claim 1, wherein the reducingdiluent gas is hydrogen.
 4. A process as set forth in claim 1, whereinthe reducing diluent gas is a mixed gas consisting of hydrogen and aninert gas.
 5. A process as set forth in claim 1, wherein the heattreatment time is 3 to 8 hours.